The present invention relates to a method and apparatus for adding a DC drive motor to a motor-generator set for driving a motion of a material handling device. In particular, this invention relates to the use of static power conversion to supply the armature current of the added DC drive motor thus eliminating the need for an extra generator to provide power to the added DC motor with the attendant problems of attempting to synchronize an added motor-generator set with the existing motor-generator set.
Material handling apparatus, for example, a dragline, have a material handling device such as a bucket or scoop suspended from an inclined boom by means of a first cable. In such case, the bucket is designed so that when it is pulled toward the base of the boom by a second cable, it will scrape soil or other material from the surface being excavated. The first and second cables are normally wound up on motor driven reels such that rotation of the respective reels in one direction or the other will determine the position of the bucket and its direction of movement. Completely independent hoist and drag machinery is used for these operations including the electrical system. In addition, a third motion, such as swing, may be used to rotate the upper portion of the apparatus with respect to its base. This motion also requires separate driving machinery. Thus, at least one driving motor is used for each motion of the material handling device such as the bucket of a dragline excavator. For instance, one or more electric motors are used to hoist the bucket and one or more separate electric motors are used to drag the bucket in a horizontal plane and one or more separate electric motors are used to rotate the upper portion of the apparatus. In other material handling devices, such as a power shovel, one or more electric motors are used to provide a crowd force for the shovel. In each of these cases, for each driving motor a separate generator is used and the generators are driven by one or more AC motors. No attempt is normally made to switch the generators from one motion motor to another motion motor. This is done because the current flow interruptions through existing electrical knife blades, contactors and so forth would cause tremendous arcing problems.
However, once the material handling apparatus or device is constructed, it is almost impossible to add an additional motor to add greater power in one or more of the motions because there must also be added to the system an additional generator and an AC motor to drive the generator. Even if space were available to add the extra DC motor, the extra DC generator and the extra AC motor to drive the generator, the approach would be very expensive.
The present invention relates to a static power conversion device which may be coupled between an additional motor and a power line source for converting the power line voltage to a variable DC armature supply voltage for the additional drive motor. Static power conversion devices are old and well-known in the art and utilize solid-state devices such as silicon controlled rectifiers, thyristors, and the like in a three-phase bridge rectifier circuit to convert the AC voltage to a DC voltage which can be used to drive an additional motor in synchronism with the existing motor-generator set to increase the power to the motion being driven by the existing motor-generator set.
Thus it is an object of the present invention to provide additional power to a motion of a material handling device by using static power conversion to obtain the necessary voltage to drive the added motor.
It is also an object of the present invention to utilize a static power conversion device for converting three-phase AC line voltage to a variable selectively reversible DC voltage and coupling the DC voltage to the armature and shunt field of an additional motor to drive the motor to assist an existing motor-generator set to drive a motion of the material handling device.